English: Hemolysis mechanism of paroxysmal nocturnal hemoglobinuria (PNH) via the complement system [4]. (a) Normal RBC possesses CD55 and CD59 which are glycosylphosphatidylinositol (GPI)-anchored self-protective complement regulatory factors. CD55 is a widely expressed membrane protein that accelerates the decay of C3 convertases. CD59 is the major inhibitor of terminal complement, which blocks the generation of the membrane attack complex (MAC). (b) Intravascular hemolysis of PNH RBC through C3 convertase and MAC. (c) Extravascular hemolysis of PNH RBC via macrophage. Eculizumab inhibits the complement activation by compensating CD59. *PNH, a life-threatening disease characterized by destruction of RBC by complement system; eculizumab, a monoclonal antibody complement inhibitor which is highly effective for PNH; C3 con, C3 convertase; C5b-8, complex of C5b, C6, C7, and C8 proteins; C3dg, a fragment of C3 protein, which is ligand of integrin (CR3) on macrophage; iC3b, inactivated C3b; Hb, hemoglobin; CR3, complementary 3.